Effect of α′ Martensite Content Induced by Tensile Plastic Prestrain on Hydrogen Transport and Hydrogen Embrittlement of 304L Austenitic Stainless Steel

Effects of microstructural changes induced by prestraining on hydrogen transport and hydrogen embrittlement (HE) of austenitic stainless steels were studied by hydrogen precharging and tensile testing. Prestrains higher than 20% at 20 degrees C significantly enhance the HE of 304L steel, as they induce severe alpha' martensite transformation, accelerating hydrogen transport and hydrogen entry during subsequent hydrogen exposure. In contrast, 304L steel prestrained at 50 and 80 degrees C and 316L steel prestrained at 20 degrees C exhibit less HE, due to less alpha' after prestraining. The increase of dislocations after prestraining has a negligible influence on apparent hydrogen diffusivity compared with pre-existing alpha'. The deformation twins in heavily prestrained 304L steel can modify HE mechanism by assisting intergranular (IG) fracture. Regardless of temperature and prestrain level, HE and apparent diffusivity (D-app) increase monotonously with alpha' volume fraction (f (alpha')). D-app can be described as log D-app = log (D(alpha')s(alpha') /s(gamma)) + log [f(alpha') /(1 - f(alpha'))] for 10% < f(alpha') < 90%, with D-alpha' is diffusivity in alpha' , s(alpha') and s(gamma) are solubility in alpha' and austenite, respectively. The two equations can also be applied to these more typical duplex materials containing both BCC and FCC phases.